JP2005321373A - Sensor and method of measuring mass flow in non-penetrating method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a sensor for measuring a mass flow in a non-penetrating method. <P>SOLUTION: The sensor comprises a probe arranged along an external wall of a tube and having a predetermined length, a heater for heating the probe at a first point, a first temperature measuring device for generating a first signal T<SB>h</SB>expressing it by measuring the temperature of the probe, a second temperature measuring device for generating a second signal T<SB>t</SB>expressing it by measuring another temperature of the probe, a third temperature measuring device for generating a signal T<SB>a</SB>expressing it by measuring a peripheral temperature, and a circuit for generating a mass flow signal on the basis of a temperature differential signal. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates generally to mass flow sensors, and more particularly to a non-intrusive sensor and method for measuring a mass flow of a medium flowing through a tube in a non-intrusive manner.

  Thermal mass flow sensors of the thermal type generally have the process of placing a heated probe through a tube wall and into a flow stream of media. The mass flow rate is a convection heat transfer coefficient related to a thermal gradient measured at two points along the length of the probe, preferably the base and tip of the probe, and Measured by its relationship to. An example of such an intrusive mass flow sensor was published on December 3, 2002, “Methods and Sensors for Mass Flow Measurement Probe Probe Heat Conduction with Method and Sensor for Mass Flow Measurement Using Heat Conduction of Probes. ) ", Which is assigned to the same assignee as the present application.

  Some applications cannot tolerate penetrating sensors, i.e. sensors having at least one probe penetrating the wall of the flow tube. In addition, in applications where the power source is limited, the power consumption of the sensor probe in the flow stream may exceed the capacity of the power source. In addition, placing a penetrating probe within the flow stream creates a pressure drop that may undesirably limit the flow of media through the tube.

Therefore, what is desirable is a non-intrusive alternative mass flow sensor and method that does not cause a pressure drop in the media flow and functions with low power consumption. The present invention provides such an alternative non-intrusive mass flow sensor and method.
US Pat. No. 6,487,904

According to one aspect of the present invention, a non-intrusive sensor for measuring the mass flow rate of a medium flowing through a tube has a predetermined length disposed along the external wall of the tube. a probe (probe), said to means for heating the probe at a first point along the said length of the probe, the temperature of the probe is measured, and generates a first signal T _h representative thereof A first temperature measuring device located at one point and another temperature of the probe is measured and along the length of the probe to generate a second signal T _t representative thereof A second temperature measuring device located at the second point and measuring and representing the ambient temperature No. T _a and the third temperature measuring device is disposed along the outer wall of the tube a distance away from the probe to generate a, and the ratio of the temperature difference signal _{(T h -T a) / (} T means connected to the first, second and third temperature measuring devices for generating a mass flow signal based on _t- T _a ).

In accordance with another aspect of the present invention, a non-intrusive method for measuring the mass flow rate of a medium flowing through a tube places a probe having a predetermined length along the outer wall of the tube. process and the steps of heating a first probe at a first point along the length of the probe, the steps of measuring the temperature T _h of the probe of the first point, second point along the length of the probe in process and the ratio of the process for measuring the ambient temperature T _a along the outer wall of a distance away the tubes from the probe, and the temperature difference measurement for measuring another temperature T _t of the probe (T _h− T _a ) / (T _t −T _a ) to measure the mass flow rate.

  A non-intrusive mass flow sensor suitable for embodying the principles of the present invention is shown in the illustration of FIG. Referring to FIG. 1, a flow tube 10 containing a flow of media is provided, which may be a gas or liquid, for example, passing through the tube in the direction of arrow 12. In this embodiment, the flow tube 10 is made of stainless steel, but it will be understood that the present invention applies to other materials as well. A probe 14 having a predetermined length is disposed along the external wall 16 of the flow tube 10. In the present embodiment, the probe 14, which may be a metal sheathed thermocouple housing, for example, is wound around the outer wall 16. The probe may be brought into thermal contact with the outer wall 16, for example, by brazing the probe to the wall 16. It will be appreciated that there are other ways of contacting the probe 14 with the wall 16 without departing from the broad principles of the present invention.

For example, a heating element 18, which may be a platinum resistive thermometer {PRT}, heats the probe 14 at a first point 22 along the length of the probe ( I ² R) is powered by the power supply 20. In this embodiment, the first point 22 is at one end or base of the probe 14. The heating element 18 may be embedded in a heat storage unit 24, which provides, for example, a copper heating block (in order to provide stabilization of the heating temperature at the first point 22). heating block of Copper). The heating block 24 is thermally coupled to the base 22 of the probe 14 and may be insulated from external temperatures to reduce heat loss. For example, a thermocouple may be, the first temperature measuring device (first temperature measuring device) 26 generates a signal indicative of measured temperature T _h of the first point 22 of the probe 14 on the wire 28 ( (general a signal over wires 28) is thermally coupled to the heating block 24.

For example, a second temperature sensing device, such as a thermocouple, is placed within the length of the probe 14 at a second point 30 along the length, which is at the second point. This is because the temperature Tt of the probe is measured and a signal representing it is generated on the wire 32. Preferably, the second temperature sensing device is located at the tip of the probe 14, which is at a substantial distance from its base. A third temperature measuring device (A third temperature measuring device) 34 is disposed along the outer wall 16 of the tube 10 by a distance 36 away from the probe 14 for measuring the ambient temperature T _a. The distance 36 may be, for example, twice as large as the diameter of the tube 10. The third temperature measuring device includes a thermocouple housing, for example, metal-wrapped around the outer wall 16, and an ambient temperature Ta to measure and generate a signal representative of the temperature on the wire 38. And a thermocouple disposed in the housing. The thermocouple housing 34 may be in thermal contact with the wall 16 by, for example, brazing.

Respective wire pairs 28, 32 and 38 from the first, second and third thermocouples are provided to a connector or connection terminal 40, where the wires are connected to differential temperature signals T _h -T _a and T _t-. Connected together to provide T _a , which will become more apparent from the following description. The signals T _h -T _a and T _t -T _a are provided to a circuit 42 for generating a mass flow signal 44 based on the ratio of the difference temperature signals (T _h -T _a ) / (T _t -T _a ). The An exemplary circuit embodiment suitable for use as circuit elements 40 and 42 in the embodiment of FIG. 1 is shown in FIG.

Referring to FIG. 5, the thermocouple wires 28 for measuring the temperature T _h is connected to the terminal 50 and 52 of the interconnection box (Interconnection box) 40, thermocouple wires 38 for measuring the temperature T _a is the box 40 The thermocouple wires 32 connected to the terminals 54 and 56 and measuring the temperature T _t are connected to the terminals 58 and 60 of the box 40. One interconnect terminals of the thermocouple T _a (interconnecting terminal) 56 is connected to the common potential of the ground or circuit 42 (common potential), T _a Another interconnect terminals 54, the temperature difference signal at the terminal 50 It is jumped to the terminal 54 of the thermocouple T _h in a manner to create a T _h -T _a (jumpered). The T _a Another interconnect terminals 54 is jumped to the thermocouple T _t of the terminal 58 in a manner to create a temperature difference signal T _t -T _a in the terminal 60.

Terminal 50 is connected to an inverting amplifier circuit 62 having a closed-loop gain of about 100. The circuit 62 includes, for example, an operational amplifier that may be an OP 200 (OP200), an input resistor R1 (1 kΩ), and a feedback resistor R2 (100 kΩ). Similarly, terminal 60 is connected to an inverting amplifier circuit 64 having a closed loop gain of about 100. The circuit 64 includes, for example, an operational amplifier which may be an OP 200 (OP200), an input resistor R3 (1 kΩ), and a feedback resistor R4 (100 kΩ). Based on the temperature difference signal ratio (T _h -T _a ) / (T _t -T _a ), the outputs of amplifiers 62 and 64 are dual matched field effect transistors to produce a mass flow signal V _O. (FET) pair and dual operational amplifier circuit configuration.

In particular, the output of amplifier 62 is through resistor R5 (10 kΩ), for example, the non-inverting (+) input of operational amplifier 66, which may be OP200, and one of FETs (F1, F2), which may be, for example, a matched pair of 2N5912. One drain of F1. The (+) input of amplifier 66 is in the virtual circuit ground. The output of amplifier 66 is connected to and drives the gates of F1 and F2, which are the pair of FETs. The source of F1 is connected to a reference voltage. Accordingly, drain or channel current I2 of F1 is proportional to the temperature difference signal T _h -T _a. Similarly, the output of amplifier 64 is connected through resistor R6 (10 kΩ) to the drain of F1 and F2 of the pair of FETs, and to the inverting (−) input of another operational amplifier 68. . The (+) input of amplifier 68 is at virtual circuit ground. The source-to-drain of FET F2 is connected between the output pair (-) input of amplifier 68. Thus, the channel current I1 F2 is proportional to the temperature difference signal _{T t} -T _a. Based on the circuit configuration described above, the output voltage V ₀ of the amplifier 68 representing the mass flow rate is equal to (I2 / I1) V _ref , which is the temperature difference ratio (T _h −T _a ) / (T _t −T _a ). Proportional.

Although the embodiment of the circuit has been described with analog circuitry, the individual temperature measurement signals T _h , T _a , and T _t are digitized in exactly the same way and then processed by a microcontroller system to determine mass flow rate. May be. An exemplary microcontroller system for this purpose is described in the above-referenced published US Pat. No. 6,057,031, incorporated herein by reference to provide more detailed details of its structure and operation.

Since the probe 14 is brazed along the outer wall 16 of the flow tube 10 and in thermal contact therewith, heat from the probe 14 is transferred through the wall 16 to the medium flowing through the tube 10. . Thus, as the media flow rate increases, the amount of heat transferred through the wall 16 increases and more heat is removed from the probe 14. Under these conditions, T _t decreases with _respect to T _h . Similarly, as the media flow rate decreases, T _t increases with _respect to T _h . In this example, T _h is not held constant and is allowed to change as long as both T _t and T _h are compensated for the ambient temperature T _a by their ratio. In addition, the separation of the thermocouple measurements T _h and T _t along the length of the probe 14 may include, for example, the heat flux of the medium, the tube material, the heat transfer of the probe, and the like. It depends on many factors.

A test was conducted on the present example under the following conditions. Thermocouple housings 14 and 34 were silver brazed to the outer wall 16 of a stainless steel flow tube 10 having a diameter of about 6.35 mm (1/4 inch). Nitrogen gas was used as the medium at room temperature and a pressure of about 0.689 MPa absolute pressure (100 psia). The heating element was two PRS configured in parallel to create a heater resistance of 74.6 Ω at room temperature and was driven with a current of 134 ma. Standard liters per minute (standard liter type device) with a critical orifice type device for measuring the actual medium flow setting (slpm). (Standard flow sensor) was used. Mass flow rate readings in the form of voltage were recorded for each setting. The test results for five exemplary flow settings are listed in the table of FIG. 3 and plotted as black dots in the graph of FIG. Referring to the table of FIG. 3, moving from left to right, the first and second columns represent the output of the standard flow sensor in volts and SLM, respectively. The third column from left to right represents the output V ₀ of the example under test. The last three columns represent a comparison between the standard flow rate reading and the flow rate reading from the example under test. The test plot of FIG. 2 yields a linear function approximated by the expression having a correlation value R ² of 0.9979: Y = 0.0565X−0.2987.

  A test was also conducted to compare the time response of this example (test sensor) with that of the standard flow sensor. FIG. 4 is a time graph illustrating such a time response comparison. Referring to FIG. 4, the ordinate is a measurement of the output voltage of each of the sensors whose voltage is expressed in SLPM, and each increment of the abscissa is the time in minutes. Represent. During the exemplary response test recorded in FIG. 4, the flow rate was changed to six different levels over time and the standard and test sensor output voltages were time recorded.

  In FIG. 4, the time response curves of the standard and test flow sensors are represented by voltage output levels L1-L6 and L1'-L6 ', respectively. In this example, the level L1-L6 of the standard flow sensor is as follows: 2.0 volts (28.31 slpm), 0.50 volts (6.964 slpm), 1.0 volts (14.05 slpm), 1.5 It corresponds to voltages and flow rates of volts (21.16 slpm), 2.0 volts (28.31 slpm), and 2.2 volts (31.18 slpm). The test flow sensor levels L1'-L6 'correspond to the following voltages: 1.3 volts, 0.065 volts, 0.52 volts, 0.92 volts, 1.3 volts, and 1.44 volts To do. Note that the test sensor responds to changes in flow rate just as well as the standard sensor. At point P10 in FIG. 4, the heater voltage of the test sensor was changed from 7 to 10 volts to determine how long the test sensor would take to recover. In this example, the test sensor recovered within about 1 minute.

The non-penetrating sensor of this embodiment results in no other pressure drop in the medium other than the pressure drop of the flow tube itself. Also, since the only elements to be heated are the heater block (which may be thermally isolated) and the probe thermocouple housing, and the heat transfer to the flow tube is very limited, the sensor has low power consumption. It works with. The reason for this is that the heat generated by the heater is sufficient to keep the heating block above the fluid temperature, and to insulate the heating block from external temperatures and provide a conduction path to the T _t sensor. This is because the heat loss may be controlled by designing it to be minimal in order to obtain a good signal. Therefore, the heat loss is substantially used for the measurement.

  Although the present invention has been described above with reference to one or more embodiments, it should be understood that such description has been presented by way of example only. Accordingly, it is not intended in any way to limit the invention to the illustrated embodiments. Rather, the present invention should be construed broadly according to the claims that follow.

1 is an illustration of an exemplary non-intrusive mass flow sensor suitable for embodying the principles of the present invention. 4 is a graph of test results for an example of the exemplary mass flow sensor. 6 is a table listing test results for examples of the exemplary mass flow sensor. 6 is a time graph illustrating the response characteristics of an example mass flow sensor example. 2 is a schematic diagram of an exemplary circuit for use in the embodiment of FIG. 1 for determining mass flow rate. FIG.

Explanation of symbols

10 Flow tube
12 Media flow direction
14 Probe
16 Exterior wall
18 Heating elements
20 Power supply
22 Base, 1st point
24 Heating block
26 First temperature measuring device
28 wires
30 Second point
32 wires
34 Third temperature measuring device
36 distance
38 wires
40 interconnect box
42 circuits
44 Mass flow signal
50, 52, 54, 56, 58, 60 terminals
62, 64, 66, 68 operational amplifier
F1, F2 FET
R1, R2, R3, R4, R5, R6 resistors

Claims (20)

A non-penetrating sensor for measuring a mass flow rate of a medium flowing through a tube, wherein the sensor comprises:
A probe having a predetermined length disposed along the outer wall of the tube;
Means for heating the probe at a first point along the length of the probe;
A first temperature measuring device disposed in the first point to generate a first signal T _h the temperature measured representative thereof of the probe,
A second temperature measuring device disposed at a second point along the length of the probe to measure another temperature of the probe and generate a second signal T _t representative thereof;
A third temperature measuring device positioned along the outer wall of the tube apart a distance from the probe to measure the ambient temperature and generates a signal T _a representative thereof,
Means connected to the first, second and third temperature measuring devices for generating a mass flow signal based on _a ratio of temperature difference signals (T _h -T _a ) / (T _t -T _a ) The sensor.   The sensor of claim 1, wherein the probe is wound around the outer periphery of the outer wall of the tube and is in thermal contact therewith.   3. The method of claim 2, wherein the first point is proximate to one end of the probe and the second point is substantially away from the one end and along the length of the probe. sensor.   The sensor of claim 1, wherein the heating means comprises a heating element in thermal contact with the probe at the first point.   2. The heating means of claim 1, wherein the heating means comprises a heat storage unit in thermal contact with the probe at the first point and a heater element for applying heat energy to the heat storage unit. The sensor. The first, second and third temperature measuring devices are connected together to provide a differential temperature signal (T _h -T _a ) and (T _t -T _a ). Of the sensor.   The sensor of claim 1, wherein each of the first, second and third temperature measuring devices comprises a thermocouple.   A thermocouple in which the probe is wound on the outer periphery of the outer wall of the tube and has a metal sheathed thermocouple housing in thermal contact therewith, and the second temperature measuring device is disposed in the thermocouple housing The sensor of claim 1, comprising:   The third temperature measuring device comprises a metal-clad thermocouple housing that is wound around and is in thermal contact with the outer wall of the tube; and a thermocouple disposed in the thermocouple housing. The sensor of claim 1 characterized in that:   The sensor of claim 1, wherein the generating means comprises an analog processing circuit. In a non-intrusive method for measuring the mass flow rate of a medium flowing through a tube, the method comprises:
Placing a probe having a predetermined length along the outer wall of the tube;
Heating the first probe at a first point along the length of the probe;
Measuring the temperature T _h of the probe at the first point;
Measuring another temperature T _t of the probe at a second point along the length of the probe;
Based on the process of measuring the ambient temperature T _a along the outer wall of the tube at a distance from the probe and the ratio of the temperature difference measurements (T _h −T _a ) / (T _t −T _a ) Measuring the mass flow rate.   2. The method of claim 1 comprising the steps of spreading the probe around the outer wall of the tube and thermally contacting the probe with the outer wall.   The method of claim 12, comprising separating the second point by a substantial distance from the first point along the length of the probe.   12. The method of claim 11, wherein the step of heating comprises the step of placing a heating element in thermal contact with the probe at the first point.   The step of heating comprises a step of placing a heat storage unit in thermal contact with the probe at the first point and a step of applying thermal energy to the heat storage unit. 11. The method of 11. The method of claim 11, wherein the step of measuring the mass flow rate comprises the step of providing a differential temperature measurements T _h -T _a and _{T t} -T _a. The ratio of the temperature difference measurement process of measuring the mass flow rate (T _h -T _a) / difference temperature measurements to provide a _{(T t -T a) T h} -T a and T _t -T _a The method of claim 16, comprising the step of analog processing. The step of placing the probe comprises a step of spreading a metal-covered thermocouple housing on the outer periphery of the outer wall of the tube and bringing the housing into thermal contact with the outer wall, wherein the temperature T _t is 12. The method of claim 11, wherein the method is measured in a thermocouple housing. The process of measuring the temperature T _a is seeded thermocouple housing is metallic exterior on the outer circumference of the outer wall of the tube comprises the step of contacting said housing outer wall and the thermal, temperature T _a is 12. The method of claim 11, wherein the method is measured in the thermocouple housing. A non-penetrating sensor for measuring a mass flow rate of a medium flowing through a tube, wherein the sensor comprises:
A thermocouple housing having a predetermined length that is wound around and is in thermal contact with the outer periphery of the outer wall of the tube;
A heating element disposed at one end of the housing for applying heat to the housing;
A first thermocouple that is disposed on one end in order to measure the temperature T _h of the housing at said one end,
A second thermocouple disposed at a point along the length of the housing, the second thermocouple disposed at the point for measuring the temperature T _t of the housing;
And comprises a third thermocouple located along the outer wall of the tube apart a distance from the housing for measuring the ambient temperature T _a,
The first, second and third thermocouples are interconnected to provide temperature difference measurement signals (T _h −T _a ) and (T _t −T _a ), and the sensor also
An analog circuit connected to the temperature difference measurement signal is provided to generate a mass flow signal based on the temperature difference signal ratio (T _h −T _a ) / (T _t −T _a ). The sensor.

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